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排序方式: 共有217条查询结果,搜索用时 62 毫秒
1.
T. N. Mamedov V. N. Duginov A. V. Stoykov I. L. Chaplygin D. Herlach U. Zimmermann V. N. Gorelkin J. Major M. Schefzik 《JETP Letters》1998,68(1):64-70
The residual polarization of negative muons in crystal silicon samples with phosphorus (P: 1.6×1013 cm−3) and antimony (Sb: 2×1018 cm−3) impurities is investigated. The measurements are made in a 1000 G magnetic field oriented in a direction transverse to the
muon spin in the temperature range 4–300 K. The relaxation rate and shift of the precession frequency in the silicon sample
with the phosphorus impurity are measured more accurately than previously. It is found that in antimony-doped silicon the
acceptor center
μ
A1 at temperatures below 30 K can be in both ionized and neutral states. The experimental data are interpreted on the basis
of spin-lattice relaxation of the magnetic moment of an acceptor center, formation of acceptor-donor pairs, and recombination
of charge carriers at the acceptor. Preliminary measurements showed a nonzero residual polarization of negative muons in germanium.
Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 61–66 (10 July 1998) 相似文献
2.
Deák L. Bottyán L. Major M. Nagy D. L. Spiering H. Szilágyi E. Tanczikó F. 《Hyperfine Interactions》2002,144(1-4):45-52
Hyperfine Interactions - Synchrotron Mössbauer Reflectometry (SMR), the grazing incidence nuclear resonant scattering of synchrotron radiation, can be applied to perform depth-selective phase... 相似文献
3.
Continuous Wave (CW) laser action has been observed from a Ho:Tm:LuAG rod at 2.1 m pumped by a diode laser at 785 nm in the temperature range of 77–200 K. The maximum power observed was 1.38 W at an incident power of 4 W at a crystal temperature of 77 K. The optical-to-optical conversion efficiency was observed to be 35.6% at a threshold power of 0.2 W with a 95% output coupling mirror. 相似文献
4.
The effects of the administration of a single dose of the model nephrotoxin mercuric chloride (2.0 mg kg(-1), subcutaneous) to male Wistar-derived rats on the urinary metabolite profiles of a range of endogenous metabolites has been investigated using (1)H NMR and HPLC-MS. Urine samples were collected daily for 9 days from both dosed and control animals. Analysis of these samples revealed marked changes in the pattern of endogenous metabolites as a result of HgCl(2) toxicity. Peak disturbances in the urinary metabolite profiles were observed (using both NMR and HPLC-MS) at 3 days post dose. Thereafter the urinary metabolite profile gradually returned to a more normal composition. Markers of toxicity identified by (1)H NMR spectroscopy were raised concentrations of lactate, alanine, acetate, succinate, trimethylamine (TMA), and glucose. Reductions in the urinary excretion of citrate and alpha-ketoglutarate were also seen. Markers identified by HPLC-MS, in positive ion mode, were kynurenic acid, xanthurenic acid, pantothenic acid and 7-methylguanine which decreased after dosing. In addition an ion at m/z 188, probably 3-amino-2-naphthoic acid, was observed to increase after dosing. As well as these identified compounds other ions at m/z 297 and 267 decreased after dosing. In negative ion mode a range of sulfated compounds were observed, including phenol sulfate and benzene diol sulfate, which decreased after dosing. As well as the sulfated components an unidentified glucuronide at m/z 326 was also observed to decrease after dosing. The results of this study demonstrate the complementary nature of the NMR and MS-based techniques for metabonomic analysis. 相似文献
5.
K. Sentrayan L. Major A. Michael V. Kushawaha 《Applied physics. B, Lasers and optics》1992,55(4):311-318
The stimulated Raman scattering (SRS) and four-wave mixing (FWM) processes have been studied in detail in CH4 gas pumped by the third-harmonic of Nd:YAG laser at =355 nm. The conversion efficiency and average output energy have been measured for the various Stokes and anti-Stokes lines at different experimental conditions. The threshold input energy at various CH4 pressures has been calculated for the unguided and capillary waveguide Raman cells. The calculated values for the capillary case have been compared with the measured values at different CH4 pressures and the two results are in good agreement. 相似文献
6.
Simulations of remote mutants of dihydrofolate reductase reveal the nature of a network of residues coupled to hydride transfer
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Daniel Roston Amnon Kohen Dvir Doron Dan T. Major 《Journal of computational chemistry》2014,35(19):1411-1417
Recent experimental and theoretical studies have proposed that enzymes involve networks of coupled residues throughout the protein that participate in motions accompanying chemical barrier crossing. Here, we have examined portions of a proposed network in dihydrofolate reductase (DHFR) using quantum mechanics/molecular mechanics simulations. The simulations use a hybrid quantum mechanics‐molecular mechanics approach with a recently developed semiempirical AM1‐SRP Hamiltonian that provides accurate results for this reaction. The simulations reproduce experimentally determined catalytic rates for the wild type and distant mutants of E. coli DHFR, underscoring the accuracy of the simulation protocol. Additionally, the simulations provide detailed insight into how residues remote from the active site affect the catalyzed chemistry, through changes in the thermally averaged properties along the reaction coordinate. The mutations do not greatly affect the structure of the transition state near the bond activation, but we observe differences somewhat removed from the point of C? H cleavage that affect the rate. The mutations have global effects on the thermally averaged structure that propagate throughout the enzyme and the current simulations highlight several interactions that appear to be particularly important. © 2014 Wiley Periodicals, Inc. 相似文献
7.
Christopher J. Major Dr. Zheng-Wang Qu Prof. Dr. Stefan Grimme Prof. Dr. Douglas W. Stephan 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(27):e202200698
The known boranes (R(Me3Si)N)2BF (R=Me3Si 1 , tBu 2 , C6F5 3 , o-tol 4 , Mes 5 , Dipp 6 ) and borinium salts (R(Me3Si)N)2B][B(C6F5)4] (R=Me3Si 7 , tBu 8 ) are prepared and fully characterized. Compound 7 is shown to react with phosphines to generate [R3PSiMe3]+ and [R3PH]+ (R=Me, tBu). Efforts to generate related borinium cations via fluoride abstraction from (R(Me3Si)N)2BF (R=C6F5 3 , o-tol 4 , Mes 5 ) gave complex mixtures suggesting multiple reaction pathways. However for R=Dipp 6 , the species [(μ-F)(SiMe2N(Dipp))2BMe][B(C6F5)4] was isolated as the major product, indicating methyl abstraction from silicon and F/Me exchange on boron. These observations together with state-of-the-art DFT mechanistic studies reveal that the trimethylsilyl-substituents do not behave as ancillary subsitutents but rather act as sources of proton, SiMe3 and methyl groups. 相似文献
8.
9.
Is it True That the Normal Valence‐Length Correlation Is Irrelevant for Metal–Metal Bonds?
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Dr. Vijay Singh Dr. Mudit Dixit Dr. Monica Kosa Prof. Dan Thomas Major Prof. Elena Levi Prof. Dr. Doron Aurbach 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(15):5269-5276
The most intriguing feature of metal–metal bonds in inorganic compounds is an apparent lack of correlation between the bond order and the bond length. In this study, we combine a variety of literature data obtained by quantum chemistry and our results based on the empirical bond valence model (BVM), to confirm for the first time the existence of a normal exponential correlation between the effective bond order (EBO) and the length of the metal–metal bonds. The difference between the EBO and the formal bond order is attributed to steric conflict between the (TM)n cluster (TM=transition metal) and its environment. This conflict, affected mainly by structural type, should cause high lattice strains, but electron redistribution around TM atoms, evident from the BVM calculations, results in a full or partial strain relaxation. 相似文献
10.
A. Major J.S. Aitchison P.W.E. Smith F. Druon P. Georges B. Viana G.P. Aka 《Applied physics. B, Lasers and optics》2005,80(2):199-201
We report on the z-scan characterization of the nonlinear refractive indices of three borate crystals (GdCOB, YCOB, and BOYS), which are new promising Yb-doped laser hosts. The results indicate the possibility of substantial Kerr-lensing and self-phase modulation effects in femtosecond oscillators and amplifiers. High values of the nonlinear refractive indices suggest possible KLM operation of such lasers. PACS 42.65.An; 42.65.Hw; 42.55.Rz; 42.65.Re 相似文献